Unix Power Tools
Handle Too-Long Command Lines with xargs

xargs is one of those UNIX utilities that seems pretty
useless when you first hear about it -- but turns into one of the
handiest tools you can have.

xargs reads a group of arguments from its standard
input, then runs a UNIX command with that group of arguments. It
keeps reading arguments and running the command until it runs out of
arguments. The shell's backquotes do the same kind of thing, but they
give all the arguments to the command at once. This can give you a
Too many arguments error.

Here are a couple of examples:

If you want to print most of the files in a large directory,
put the output of ls into a file.
Edit the file to leave just the filenames you want printed.
Give the file to xargs' standard input:

% ls > allfiles.tmp
% vi allfiles.tmp
% xargs lpr < allfiles.tmp

What did that do? With lines like these in
allfiles.tmp:

% cat allfiles.tmp
afile
application
...
yoyotest
zapme

xargs ran one or more lpr commands, each
with a group of arguments, until it had read every word in the
file:

lpr afile application ...
...
lpr ... yoyotest zapme

The standard output of xargs is the standard
output of the commands it runs.
So, if you'd created allfiles.tmp above but you wanted to format
the files with pr
first, you could type:

% xargs pr < allfiles.tmp | lpr

Then xargs would run all of these pr
commands. The shell would pipe their standard outputs to a single
lpr command. Actually, the shell is piping the standard
output of xargs. As I said above, xargs
sends the standard output of commands it runs to its own standard
output:

pr afile application ...
...
pr ... yoyotest zapme

In this next example,
find
gets a list of all files in the directory tree.
Next, we use xargs to read those filenames and run
grep - l
to find which files contain the word "WARNING".
Next, we pipe that to a setup with pr and lpr,
like the one in the previous example:

% find . -type f -print | xargs grep -l WARNING | xargs pr | lpr

"Huh?" you might say. Just take that step by step. The output of
find is a list of filenames, like ./afile ./bfile ... ./adir/zfile and so on. The
first xargs gives those filenames to one or more
grep - l commands:

The standard output of all those greps is a (shortened)
list of filenames that match. That's piped to another
xargs - it runs pr commands with the
filenames that grep found.

UNIX is weird and wonderful!

Sometimes you don't want xargs to run its
command with as many arguments as it can fit on the command line.
The - n option sets the maximum number of arguments
xargs will give to each command. Another handy option,
- p, prompts you before running each command.

Here's a directory full of files with errors (whose names end with
.bad) and corrected versions (named .fixed).
I use ls to give the list of files to xargs;
it reads two filenames at once, then asks whether I want to run
diff - c to compare those two files. It keeps prompting
me and running diff - c until it runs out of file
pairs: